Learning Check Point

Measuring Sea Level Change

Please take a few minutes to think about what you just learned, then answer the questions below to test your knowledge.

Question 1 - Multiple Choice

Sea level position can vary in the short-term and in the long-term. In this case, short-term or periodic means:

a. sea level change observed in the last century.
b. sea level change from events like storm surge, or tides, with time periods ranging from a few hours (i.e., tsunami) to a few days.
c. sea level change due to processes like subsidence and tectonic activities that take place over centuries to millennia or longer.
d. none of these accurately defines this term.

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Question 2 - Multiple Choice

If one location on the coast of Maine or Svalbard in the North Atlantic shows evidence of localized sea level fall with emergent cliffs and shorelines, while locations like the Gulf Coast of the U.S. or the river deltas in Bangladesh show evidence of sea level rise as barrier islands flood and erode.

Which of the following provides the best explanation to reconcile these observations?

a. Absolute sea level rise is taking place and data sensors measuring sea level position in Maine and Svalbard must not be working properly.
b. Physical evidence of coastal change (like emergent cliffs or drowned coastlines) cannot be used as evidence of sea level change because tectonic processes are constantly changing the level of the land and the shape of ocean basins.
c. Global or eustatic sea level is an average of sea level positions around the globe relative to a fixed datum on land, so some locations will show evidence of lowering of sea level while others will show evidence of sea level rise.
d. Relative sea level for any location will be dependent on local factors controlled by the geologic setting, geographic aspect, oceanographic currents, and more.

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Question 3 - Multiple Choice

Geoscientists use most of the following to measure and detect sea level elevations so that change rates can be calculated or so that tsunami waves can be detected.

Which is not currently used to measure sea level elevation?

a. tide stations outfitted with water-level measurement sensors
b. satellite data that measures sea surface elevation
c. ship board depth sounders
d. floating buoys

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Question 4 - Multiple Choice

Scientists and government agencies have been collecting sea level measurements routinely for less than a century, in most cases. One site in the U.S,. i.e., Fort Point in San Francisco, California has a century-long record.

The sea level measurements from this station have been studied and show evidence of all of the following except:

a. long-term trend over decades of sea-level rise.
b. influence of events like El Niños and La Niñas.
c. evidence of sunspot activity.
d. daily and annual sea-level cycles.

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Question 5 - Multiple Choice

Coastlines are not straight and are often irregular in shape with promontories and embayments. When oceanic and atmospheric processes combine and interact with shorelines this can produce drastic differences in the impacts of coastal processes from one location to the next on the same shoreline.

Based on this and your understanding from this module thus far, which of the following conditions would likely contribute to the highest storm surge levels for the location described.

a. onshore-directed winds focused into a narrrowing embayment or funnel-shaped coastline.
b. onshore-directed winds focused onto a peninsula or coastal promontory with high cliffs and a narrow shelf.
c. shore-parallel winds focused along a long wide coastal shelf.
d. offshore-directed winds moving down a steep high-gradient coastline.

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Discussion

Modern sea level is not a simple, straight-forward concept. Sea level today is a snapshot of longer-term patterns in sea level change that are incredibly dynamic (stochastic). It should be clear that sea level, as observed today, is a product of many different factors. Each factor can amplify (add to) or attenuate (subtract from) other factors. So, sea level position at any location is like your paycheck after taxes, dues, penalties, and other deductions are assessed. Your gross salary can be "predicted" based on your salary/hourly wage, but your net take-home pay depends on any bonuses (additive) or deductions (subtractive) assessed. So, at any point, your net pay, like the observed sea level, can be slightly different, if not substantially different, depending on events that unfold from day to day, month to month, and year to year.

The case example from San Francisco illustrates the fact that average water levels that are used to produce the "predicted" water level algorithm and output actually over-estimated water levels for the interval where actual observations were made. We determined that water levels were lower than predicted, and we asked why that might be the case.

Clearly, the ~18 hour interval showed a "stochastic event" that influenced the periodic trend - the same event was also recorded at other stations in the region, but not others at distance on the same coastline, i.e., in Washington. These observations suggest the event may have been related to short-term meteorological variable.